There are a series of electrophoretically different hemoglobins appearing in sequence during development of the chick embryo. The mechanism which elicits such transitions in hemoglobins is not clear. Our studies are directed toward the ultimate goal of elucidating the control mechanism of differentiation in the developing erythropoeitic system. Recent work in our laboratory has shown the presence of three DNA polymerase activities in these developing erythrocytes - two activities found primarily in the cytoplasm and one in the nucleus. All three polymerase activities vary according to the age of the chick embryos such that on the fifth day of chick embryogenesis there is a marked decline of activities and remains at the low level through day 9. In addition, since the discovery of poly A in mRNA, we have investigated the biosynthesis of poly A rich RNA in erythrocytes of developing chick and have found a positive correlation between the timing of the synthesis of such type of RNA and the switching of hemoglobin types. Furthermore, we have detected several enzymatic activities which catalyze the polymerization of adenylate. These enzymatic activities also vary according to the age of the embryos such that there is a parallel between the timing of poly A rich RNA synthesis and the enzymatic activities.